93 



CHAPTER XIII. 



MICROSCOPICAL PLANTS. 



Uncertain Limits between the Animal and the Vegetable "World. The simplest 

 Forms of Plants. Protococci. Oscillatorise. Volvocinse. Desmidiae. Dia- 

 tomacese. Their Importance in the Household of the Seas. Their Geological 

 Agency. 



THE limits between the vegetable and the animal world are 

 by no means so strictly denned as might be supposed when 

 merely considering the higher classes of both kingdoms. No 

 one can possibly doubt the vegetable nature of the tree which 

 he sees firmly rooted in the soil, or be inclined to reckon the 

 swift-winged bird among the plants ; but in the lowest and small- 

 est forms of organic life, spontaneous motion ceases to be the 

 distinctive character of animality. For the microscope has 

 taught us not merely that the spores of the algae, and many of 

 the minutest plants, possess a power of spontaneous movement, 

 but also that the instruments of motion, when these can be dis- 

 covered, are of the very same character in the plant as in many 

 of the lower animals, being little hair-like filaments, termed cilia 

 (from the Latin cilium, an eyelash), by w-ljose rhythmical 

 vibrations the body of which they form a part is propelled in 

 definite directions. The peculiar contractility of these cilia 

 cannot be accounted for in either case, any better than in the 

 other ; all we can say is, that it seems, in all probability, to de- 

 pend upon the continued vital activity of the living substance of 

 which these filaments are prolongations, and that this contractile 

 substance has a composition essentially the same in the plant as 

 in the animal. Thus, in the present state of our knowledge, it 

 is very difficult to lay down any definite line of demarcation 

 between the two kingdoms; the only character which appears 

 to establish a difference being that the simplest animals, like 

 the highest members of their class, depend for nutriment upon 



